Crystal packing in six crystal forms of pancreatic ribonuclease.

We compare the molecular packing of bovine pancreatic ribonuclease A (RNase A) in six crystal forms, two grown with alcohol, three with high salt and one with polyethylene glycol as a precipitant. The six packings differ in the number of molecules in contact and in the extent of the contacts, which bury 1570 A2 to 2790 A2 of the RNase surface. Regions of the protein surface involved in the six packings cover almost the whole RNase molecule. The abundance of polar interactions, about one per 200 A2, is the same in all types of precipitants. All molecule-to-molecule contacts are different in the six crystal forms, except for the one that forms a RNase dimer. The dimer has a large interface covering 1800 A2 and eight to ten polar interactions. Its presence in the three salt-grown crystal forms suggests that it is an intermediate in salt induced crystallization. In contrast, the two alcohol-grown forms contain only small interfaces, implying a different mechanism of nucleation.